Over the holidays I took my children to get their flu shots at our pediatrician’s office. Only one parent was allowed to go into the office with them. My wife and I left it up to the kids to choose who they wanted. And they chose their mother. I’m the doctor, and yet they chose their mother!

In any event, the experience allowed me to spend some time in an area of the medical profession I see only rarely: the waiting room. Many of the children around me exhibited the red nose and frequent sneezing of those suffering from the common cold. And I noticed something: All of the small children were coughing into their sleeves.

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Years ago, parents would tell kids to use their hands to cover their mouths when they coughed. Then those children would use the same hand to touch subway poles, escalator railings or door handles, transmitting the germs they just coughed out. Then came “sleeve sneeze” public health campaigns. So far as I can tell through media database searches, Toronto’s began in 2006, amid increasing concerns about the pandemic flu. Whenever it started, the “sleeve sneeze” campaign must be one of the great public health victories of the early 21st century: Everyone seems to use it.

I do wonder, however, whether we’re all becoming a bit too paranoid about germs. I include my own family in this group. Once we left the doctors’ office, for example, my wife and I encouraged our children to use a hand sanitizer. When our kids were toddlers our house had alcohol wipes and Purell vials all over the place. But is all this washing and disinfecting really necessary? Is it proactive prevention? Or overly paranoid fear?

That, at least, is the thinking behind the “hygiene hypothesis,” a school of thought first proposed by David P. Strachan in 1989, and now experiencing a resurgence that’s probably a response to society’s mania for cleanliness. Strachan’s original study sought to explain why British kids with greater numbers of older siblings had fewer incidences of hay fever, speculating that perhaps it could be the fact kids with lots of older siblings tend to be exposed to greater numbers of germs. While it was greeted with skepticism early on, Strachan’s theory has since been confirmed. In fact, in the decades since, greater exposure to germs early in life has also been associated in epidemiological studies with lower levels of asthma, some allergies and even such autoimmune diseases as type-1 diabetes and multiple sclerosis.

For example, a study published in the New England Journal of Medicine in February 2011 found that kids who grew up on the comparatively microbe-rich environment of rural farms “had lower prevalence’s of asthma.”

The paper analyzed two large, longitudinal studies based on more than 16,000 elementary school children located in south Germany, Switzerland and Austria. The researchers collected and analyzed dust from the children’s homes. They found that dust from farming homes contained more microbes than suburban homes, which stands to reason. What’s more, researchers believe this explains the farm kids’ significantly lower incidence of asthma. The remarkable thing? How big the difference was; in one of the studies, farm kids had half the asthma rates of the greater population.

“These data support the idea that the greater diversity of microbial exposure among children who live on farms is associated with the protection from the development of asthma,” study researchers reported, speculating that microbial exposure may encourage development of immune system cells that in turn suppress the production of the sort of immune-system cells that trigger asthmatic reactions. Researchers’ next hope to determine which microbes are most responsible for preventing asthma — and that, perhaps, may lead to new therapies, such as targeted microbe exposures, for the dreaded respiratory malady.

More broadly, the study is a reminder that humans have been living and fighting off germs for tens of thousands of years. Particularly when we’re young, germs serve an important purpose for the development of the immune system. By depriving our children of exposure to germs, we may be depriving them the benefits of a process the human body has evolved over aeons, a process that helps to create healthy and allergy-free adults.

I’m not suggesting we give up the sleeve sneeze. Nor am I suggesting that parents encourage their kids to go out and intentionally expose themselves to germs, by licking the surfaces in subway cars or attending chicken-pox parties. As in most things, it’s balance that’s important. Maybe we recognize we’re becoming a bit paranoid about germ exposure. Maybe we pare back a little bit on the Purell and the hand-washing (unless the risk of significant infection is high). After all, germs are one of those things that complies with the old adage: What doesn’t kill us often makes us stronger.

Dr. James Aw is the medical director of the Medcan Clinic, a leading private health clinic in Toronto. For more information, visit medcan.com.